1. Technical Field
The present invention relates to graphite articles, and a process for preparing the inventive graphite articles. More particularly, the invention concerns articles such as graphite electrodes or cathodes formed by processing a blend of (i) a particulate fraction comprising at least about 35 weight percent calcined coke and (ii) pitch, where the blend further includes small particle size filler, carbon fibers, or combinations thereof.
2. Background Art
Graphite electrodes are used in the steel industry to melt the metals and other ingredients used to form steel in electrothermal furnaces. The heat needed to melt metals is generated by passing current through a plurality of electrodes, usually three, and forming an arc between the electrodes and the metal. Electrical currents in excess of 100,000 amperes are often used. The resulting high temperature melts the metals and other ingredients. Generally, the electrodes used in steel furnaces each consist of electrode columns, that is, a series of individual electrodes joined to form a single column. In this way, as electrodes are depleted during the thermal process, replacement electrodes can be joined to the column to maintain the length of the column extending into the furnace.
Generally, electrodes are joined into columns via a pin (sometimes referred to as a nipple) that functions to join the ends of adjoining electrodes. Typically, the pin takes the form of opposed male threaded sections, with at least one end of the electrodes comprising female threaded sections capable of mating with the male threaded section of the pin. Thus, when each of the opposing male threaded sections of a pin are threaded into female threaded sections in the ends of two electrodes, those electrodes become joined into an electrode column. Commonly, the joined ends of the adjoining electrodes, and the pin there between, are referred to in the art as a joint.
Given the extreme thermal stress that the electrode and the joint (and indeed the electrode column as a whole) undergoes, mechanical/thermal factors such as strength, thermal expansion, and crack resistance must be carefully balanced to avoid damage or destruction of the electrode column or individual electrodes. For instance, longitudinal (i.e., along the length of the electrode/electrode column) thermal expansion of the electrodes, especially at a rate different than that of the pin, can force the joint apart, reducing effectiveness of the electrode column in conducting the electrical current. A certain amount of transverse (i.e., across the diameter of the electrode/electrode column) thermal expansion of the electrode in excess of that of the pin may be desirable to form a firm connection between pin and electrode; however, if the transverse thermal expansion of the electrode greatly exceeds that of the pin, damage to the electrode or separation of the joint may result. Again, this can result in reduced effectiveness of the electrode column, or even destruction of the column if the damage is so severe that the electrode column fails at the joint section. Thus, control of the thermal expansion of an electrode, in both the longitudinal and transverse directions, is of paramount importance.
As a consequence, if the pin can be eliminated from the electrode/electrode column system, the need to balance the thermal expansion of the different system components (i.e., pin and electrode) is reduced. Prior attempts to eliminate the pin have been attempted, where a threaded electrode end or other electrode mating means have been employed. Industry acceptance has lagged, however, since it is felt that the strength of the graphite is not sufficient to maintain the integrity of the electrode column without a pin. Regardless of whether the pin is eliminated or not, increased graphite electrode strength and toughness (which can be defined as resistance to cracking) and reduction of brittleness (which can be defined as the rate of propagation of cracks) is desired in order to extend electrode life.
Similarly, in the case of graphite cathodes (utilized in the aluminum smelting industry) and other synthetic graphite artifacts, increased strength and toughness will result in longer life and improved usability.
There have been references to the use of mesophase pitch-based carbon fibers to improve specific properties of bulk graphite products such as electrodes. For instance, Singer, in U.S. Pat. No. 4,005,183, describes the production of mesophase pitch-based fibers and states that, because of their low electrical resistivity, these fibers can be employed as filler material in the production of graphite electrodes. In British Patent 1,526,809 to Lewis and Singer, 50% to 80% by weight of carbon fibers are added to 20% to 50% by weight of pitch binder and then extruded to form a carbon artifact that can be graphitized. The resulting article exhibits relatively low longitudinal thermal expansion.
In U.S. Pat. No. 4,998,709, Griffin et al. attempt to address the problems caused by excessive longitudinal thermal expansion of electrode pins by preparing a graphite nipple (i.e., pin) with mesophase pitch-based carbon fibers included in the extrusion blend. The carbon fibers used by Griffin et al. have a Young's modulus of greater than 55×106 pound-forces per square inch (psi), and are present in the blend at about 8 to 20 weight percent. The blend is extruded, baked, and then graphitized for from about 5 to 14 days to produce the nipple. Although nipples produced by the Griffin et al. process show a decrease in the coefficient of thermal expansion (CTE) in the longitudinal direction, they also show an undesirable increase in CTE in the transverse direction, an increase in electrical resistivity and a decrease in the modulus of rupture. In addition, the graphitizing time is extremely long compared with times that would be advantageous for commercial production.
In an improved process for preparing connecting pins containing fibers, Shao et al. teach the inclusion of carbon fibers derived from mesophase pitch in the calcined coke/pitch blend, in U.S. Pat. No. 6,280,663. The resulting pins exhibit reduced longitudinal CTE without requiring commercially disadvantageous graphitizing time. However, even such improved pins as produced by the Shao et al. process do not eliminate the need for electrodes with improved strength; additionally, if pins could be eliminated altogether, the savings and efficiency gains would be extremely beneficial.
What is desired, therefore, is a graphite article having reduced CTE in the longitudinal direction as compared with art-conventional graphite articles, without sacrificing transverse CTE or resistivity and modulus of rupture. Moreover, graphite articles having increased strength and toughness, especially increased strength and toughness sufficient to permit mating of electrodes without the use of a pin, are also desired. It is also highly desirable to achieve these property benefits without using high quantities of expensive materials.